Directed: Ortho C-H borylation catalyzed using Cp?Rh(iii)-NHC complexes

Article abstract of DOI:10.1039/c8cc03144d

Cp?Rh(NHC) complexes with bulky chiral bidentate NHC-carboxylate ligands were efficiently synthesized and fully characterized including solid-state structures. These unprecedented rhodium(iii) complexes demonstrated high selectivity in pyridine-directed ortho-C-H borylation of arenes under mild conditions.

Full text of DOI:10.1039/c8cc03144d

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Directed Ortho C-H borylation catalyzed by Cp*Rh(III)-NHC
complex
Received 00th January 20xx,
Accepted 00th January 20xx
Jompol Thongpaen,a Thibault Schmid,a Loic Toupet,b Vincent Dorcet,c Marc Mauduita and
Olivier Baslé*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
Cp*Rh(NHC) complexes with bulky chiral bidentate NHC- the directed C-H borylation of arenes.11 The present report
carboxylate ligands were efficiently synthesized and fully describes the pyridine-directed regioselective ortho-C-H
characterized including solid-state structures. These borylation of arenes catalyzed by newly developed
unprecedented rhodium (III) complexes demonstrated high (NHC)Cp*Rh(III) complexes under mild reaction conditions.
selectivity in the pyridine-directed ortho-C-H borylation of
Recently, we described a practical multicomponent
arenes under mild conditions.
strategy that provides access to various unsymmetrical NHC
ligands precursors, including carboxyalkyl-NHCs that are
beyond reach otherwise.12 NHC ligands containing an
additional donor functionality have demonstrated
significant benefit in stabilizing a variety of transition-metal
complexes.13 Therefore, we decided to evaluate our new
chiral bidentate NHCs as ligands for the synthesis of stable
Rh(III) complexes bearing the relatively electron rich and
sterically demanding Cp* ligand.14
Transition metal-catalyzed direct C-H bond borylation is
among the most important synthetic route to organoboron
compounds that are versatile intermediates in medicinal
chemistry and material science.1 While the selectivity of the
arene functionalization was mainly governed by steric
hindrance for original catalytic systems,2 considerable
interest has been recently given to the development of
efficient catalyst for the regioselective ortho-borylation
assisted by directing-groups.3 In this concern, high
efficiency has been achieved with a variety of transition
metals in association with various phosphine and nitrogen-
based ligands.4 On the other hand, and despite the
tremendous success of
N-heterocyclic carbene (NHC)
ligands in homogeneous catalysis,5 the use of NHC ligand in
transition metal-catalyzed C-H bond borylation is scarce,6
and reports involving regioselective ortho-borylation with
NHC/metal catalyst are even more rare.7 In the last decade,
Rh(III)-catalyzed C-H functionalization of arene has
attracted increasing attention, especially with the
Scheme 1 Synthesis of Cp*Rh(NHC) complexes with bulky chiral bidentate NHC ligands.
NHC Ligands precursors were readily prepared by
simply mixing mesitylamine, an amino-acid, formaldehyde
and glyoxal in acetic acid for few minutes (Scheme 1).12b
development
of
efficient
Cp*Rh
Starting from
1c), we could obtain the corresponding unsymmetrical
imidazolium salts (2a 2b, 2c) with good selectivity and
L-leucine (1a), L-valine (1b) and L-alanine
(pentamethylcyclopentadienyl rhodium)-based catalytic
systems for regioselective C-C and C-heteroatom bond
formation assisted by directing-group.8 While Cp*Rh(III)
complexes are recognized as the most active catalysts for
the C-H borylation of alkanes,9 including methane,10 much
less interest has been given to this family of complexes in
(
,
satisfactory 52%, 46% and 42% yield, respectively. The
chloride salts were subsequently reacted with Ag2O to form
the olig- or polymeric carbene silver intermediates that
were subsequently engaged in a transmetallation reaction
with [Cp*RhCl2]2 affording the corresponding chelating
(NHC)Cp*RhCl complexes (3a, 3b, 3c) in excellent 92%,
82% and 94% yield, respectively. The 13C NMR
spectroscopy analyses evidenced doublet peaks at δc 170.7
ppm (1JRh-C = 54.7 Hz), c 170.1 ppm (1JRh-C = 55.0 Hz) and δc
δ
169.5 ppm (1JRh-C = 54.5 Hz) characteristic of the 103Rh
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coupling with the carbenic carbon of 3a
,
3b and 3c
,
the chelating NHC ligand for the catalyst activity. Finally,
respectively.14 According to H NMR analysis, complexes 3a control experiments confirmed that the borylation reaction
and 3b were formed in two inseparable diastereomers (d.r. did not occur in the absence of base or rhodium complex
1
= 9:1); and only the solid-state structure of the (
S)c(
S
)
(entry 16-17).
Rh
Table 1 Optimization of Reaction Conditiona
isomers were determined by X-ray crystallography analyses
(Figure 1). According to the crystal structure, these
rhodium complexes exhibit
a three-legged piano-stool-
typed geometry, which is commonly observed in half-
sandwich complex,15 and the bond length between Rh and
NHC was measured to be 2.067 Å in 3a and 2.056 Å in 3b
.
On the other hand, a decrease of the diastereoselectivity
was observed in the synthesis of complex 3c (d.r. = 75:25)
that is attributed to the reduced chiral information of the
Entry
Cat.
Base
Solvent
Temp.
Yield (%)b
1
2
3a
KOAc
K2CO3
toluene
toluene
toluene
toluene
toluene
THF
80
80
80
80
80
80
80
80
70
r.t.
80
80
80
80
80
80
80
0
8
3a
alanine based NHC ligand 2c
.
3
3a
NaOtBu
NaOiPr
NaOMe
NaOMe
NaOMe
NaOMe
NaOMe
NaOMe
NaOMe
NaOMe
NaOMe
NaOMe
NaOMe
-
12
61
71
34
9
4
3a
5
3a
6
3a
7
3a
n-octane
benzene
benzene
benzene
benzene
benzene
benzene
benzene
benzene
benzene
benzene
8
3a
86
43
n.r.
20
80
54
25
29
n.r.
n.r.
9
3a
10
11c
12
13
14
15
16
17
3a
3a
3b
Figure 1 X-ray crystal structure of 3a (left) and 3b (right). Hydrogen atoms are omitted
for clarity.
3c
[Rh]e
[Rh]e/IMes
With those new complexes in hand, we decided to
evaluate their potential as catalysts in the C-H bond
3a
-
NaOMe
borylation
of
2-phenylpyridine
using
B2pin2
a
Reaction conditions: 2-phenylpyridine (0.2 mmol), B2pin2 (0.4 mmol), Catalyst
[bis(pinacolato)diboron] as borylating reagent (Table 1).
First, the base screening was performed in toluene at 80 C
b
(0.01 mmol of Rh), base (0.02 mmol), solvent (2 mL), 16 hrs, under Ar. NMR
°
c
yield using 1,3,5-trimethylbenzene as an internal standard. HBpin was used
instead of B2pin2. e [Rh] = [Cp*RhCl2]2. n.r. = no reaction, r.t. = room temperature.
in the presence of complex 3a as catalyst. While no reaction
was observed with potassium acetate, the desired product
could be formed in modest yields in the presence of
potassium carbonate and sodium tert-butoxide (entries 1-
3). To our delight, a decrease of the bulkiness of the
alkoxide base resulted in an increase of the catalytic
activities to afford the borylated product in up to 71% NMR
yield when using sodium methoxide (entry 5). Then, lower
catalytic activities were observed when toluene was
To test the viability of this method, we first turned our
attention to other 2-arylpyridine based-substrates (Table
2). The yield of the ortho-borylated product
5 was
determined by 1H NMR spectroscopy and subsequent
quantitative oxidation with Oxone® allowed efficient
isolation of the corresponding hydroxylated product 6 2b
.
replaced by tetrahydrofuran or n-octane (entries 6 and 7).
Under the optimized reaction conditions with catalyst 3a
,
The optimized reaction conditions were found using
benzene as solvent at 80 °C with 10 mol% of NaOMe,
affording the borylated product in 86% yield (entry 8). In
fact, a decrease of the reaction temperature to 70°C has
halved the yield (entry 9), and no reaction was observed at
room temperature (entry 10). When pinacolborane (HBpin)
was used instead of B2pin2, the desired product could only
be obtained in modest 20% yield (entry 11). It is also
important to note that under the optimal reaction
conditions, complex 3b could catalyzed with similar
substrates bearing an ether electron-donating (4b and 4c
)
and an ester electron-withdrawing para-substituents (4d
and 4e) behave comparably to the parent substrate 4a. A
slight decrease of reactivity was observed with 4e bearing a
trifluoromethane group in the para position. On the other
hand, while electronic effects may be neglected, the reaction
was sensitive to the steric environment at the aryl ring.2,7 In
fact, the borylation process was considerably affected by a
methyl substituent in the ortho or meta position and thus
affording the products 5g and 5h with low 10 and 25%
yield, respectively. Moreover, the borylation of (2-
naphthylene-1-yl)pyridine 4i did not take place. 2-
efficiency the borylation reaction (entry 12), while
a
noticeable decrease of catalytic activity was observed with
the aniline based complex 3c (entry 13). On the other hand,
[Cp*RhCl2]2 exhibited only poor activity to afford product
5a in 20% yield (entry 15). Moreover, the use of the
monodentate IMes NHC had no beneficial effect on the
reaction (entry 14), further highlighting the importance of
phenylpyridine having a methyl (4j), trifluoromethyl (4k
)
or phenyl substituent (4l) on the pyridyl group were also
tolerated. The isoquinoline-based substrates were effective
in ortho borylation,16 affording the functionalized products
(
6m
-
6p) in good yields. A decrease of activity was observed
2 | J. Name., 2012, 00, 1-3
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with 2-benzylpyridine-based substrates that allowed poor could be isolated or clearly identified from stoichiometric
conversion with the prochiral bulkier substrate 4r affording reaction in the presence of substrate 4a 22
Several other
.
the racemic product 6r in low yield.4i Finally, the ferrocenyl mechanisms can be envisioned, such as dissociation of the
group was ineffective in C-H borylation and did not allow NHC-carboxylate ligand to generate high oxidation state
the synthesis of the planar chiral borylated product 5s
.
Cp*RhV boryl complexes that are established catalysts in C-
H borylation.9 Nevertheless, neither the [Cp*Rh(H)2(Bpin)2]
nor the [Cp*Rh(H)(Bpin)3] with characteristic signals in 1H
and 11B NMR could be detected from the crude of
stoichiometric reactions. Furthermore, while rhodium(V)
boryl complex were recently established as efficient
catalysts in C-F bond borylation,23 our catalytic system
appeared totally inefficient in the ortho borylation of 2-
a
Table 2 Scope of substrates
(perfluorophenyl)pyridine (Scheme S2, ESI†). In addition,
the deuterium kinetic isotope effect value of 1.3 determined
in the cross-experiment using 4a and its pentadeuterated
analogue
d
5-4a indicated that the ortho C-H bond cleavage
. Finally,
was not the rate-limiting step (Fig. S4, ESI
†
)
additional experiments in the presence of butylated
hydroxytoluene (BHT) and mercury metal suggested that
catalytic processes involving radicals or heterogeneous
metal are unlikely to be operative. Despite these
observations, the exact mechanism through which
complexes
3 catalyze the pyridine-directed regioselective
ortho-C-H borylation of arenes remains uncertain.
Finally, we decided to evaluate the potential of complex
3a to catalyze alkane functionalization.9 While no reaction
was observed under the standard conditions at 80 °C, the
desired regiospecific borylation of n-octane proved effective
at 150 °C, affording 1-octylBpin in high 86 % isolated yield.
Analysis of the isolated borane product by gas
chromatography-mass spectroscopy confirmed the high
selectivity for terminal C-H bond functionalization with 1-
octylBpin being the only isomer detected.
a Reaction conditions: 4 (0.5 mmol), B2pin2 (1.0 mmol), 3a (5 mol%), NaOMe (10
b
mol%), benzene (5 mL), 16 h, under Ar; then Oxone (1.0 mmol). NMR yield of
the borylated product 5. c Isolated yield of the hydroxylated product 6.
To gain some mechanistic insight into the present
reaction, the following experiments were conducted. First,
the kinetic study of the reaction performed in C6D6, revealed
a relatively long induction period (45-60 min under the Scheme 2. Complex 3a catalyzed terminal borylation of n-octane
optimized conditions), followed by a striking color change
from dark red to dark green that is concomitant with the
initiation of the catalytic C-H borylation process (Scheme
In conclusion, an efficient strategy for the synthesis of
Cp*Rh(III)-based complexes bearing bulky chiral bidentate
NHC-carboxylate ligands was developed. The newly
synthesized Cp*Rh(NHC) complexes demonstrated catalytic
activities in the pyridine-directed regioselective ortho-C-H
borylation of arenes. The selective catalytic process was
found to be applicable to a broad range of pyridine based
substrates under mild reaction conditions. Further
investigations on the application of the chiral-NHC/Rh
system to asymmetric C-H activation reactions and to
elucidate the reaction mechanism are currently underway
in our laboratory.
We are grateful to the CNRS, the Ecole Nationale
Supérieure de Chimie de Rennes. This work was supported
by the Agence Nationale de la Recherche (ANR-15-CE07-
0012-01 “CHADOC” grant to OB and JT), the region Bretagne
(SAD n° 8734 grant to MM and TS).
S1, ESI
†). Interestingly, the intermediacy of a metal-hydride
was probe by NMR experiments. The reaction of
a
stoichiometric amount of complex 3a, NaOMe and Bpin-
Bpin at 80 °C in C6D6 led after 45 min to a dark green
solution that allowed the observation of a Rh-H resonance
by 1H NMR at
δ −10.90 (d, JRhH = 24 Hz, Fig. S3, ESI†
).18
Moreover, mass and 1H NMR spectroscopy analyses of the
crude reaction also allowed the detection of Cp*H.
Reductive elimination of Cp* with hydride ligands has been
previously observed from Cp*RhIII hydride complexes,19
with possible dissociation of the free Cp*H diene.20 Based
on the above results, we surmise that the Rh-H intermediate
originated from complex 3a could undergo Cp* ligand
dissociation allowing liberation of coordination sites to
accommodate arylpyridine substrates for ortho C-H
borylation.20 However, no cyclometalated intermediate
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Conflicts of interest
The authors declare no conflict of interest
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